This invention relates generally to active optical systems, and more particularly, to interferometers used in the measurement and compensation of aberrations in optical wavefronts. The term "active optics" applies to optical components whose characteristics are adjusted during actual operation, to control optical wavefronts. The term "optical ray" represents the direction of propagation of radiation, and a "wavefront" is a three-dimensional surface of constant optical path length, orthogonal to a family of rays emanating from a source of radiation. To form an image of a point source of radiation, all rays in the family must have the same optical path length. In a medium of constant refractive index, this is achieved by generating a spherical wavefront, or if the point source is located at infinity, a planar wavefront. Although the geometric concepts of optical rays and wavefronts have no real physical existence, they are invaluable in the design and understanding of optical systems, and will be used in this specification.
The normal function of lenses and mirrors is to adjust optical path lengths to produce desired wavefronts. Although lenses or mirrors can be moved relative to each other to effect changes in an optical system in real time, such as in a zoom lens, there are some wavefront changes that cannot be easily made in this manner. In many applications, a normally spherical or planar wavefront may be subject to variable aberrations. For example, an astronomical telescope receiving light through the atmosphere will be subject to time-varying atmospheric conditions that distort the wavefronts arriving at the telescope. Similarly, light generated by a laser device may be aberrated by variable conditions within a lasing cavity of the device. Accordingly, in any application of optics in which wavefronts are subject to significant time-varying aberrations there is a need to detect such aberrations, and in some cases to compensate for them in real time, i.e. to remove the effects of the aberrations immediately so that the optical system can function continuously as though the aberrations never existed.
Active optical systems have been developed in recent years to provide solutions to problems of this type. Although the field of active optics is relatively new, there have been many publications detailing the work performed in the area. A good survey of the state of the art, as of the year 1978, may be found in a paper by John W. Hardy, "Active Optics: A New Technology for the Control of Light," Proc. of the IEEE, Vol. 66, No. 6, pp. 651-97 June 1978. More recent developments in the field, specifically in wavefront sensing techniques, are described in papers published as the Proceedings of SPIE, the International Society for Optical Engineering, Vol. 351 Wavefront Sensing, August 1982.
Many prior-art active optical systems employ shearing interferometers to obtain a measure of wavefront distortion. Shearing interferometers are a class of instruments in which a wavefront to be measured is divided in amplitude into two components, which are then mutually displaced, and subsequently recombined to produce an interference pattern. In a lateral shear interferometer, the two components are laterally displaced with respect to each other to produce a constant unidirectional shear across the wave front. The resulting interference pattern provides an indication of the slope of the wavefront in the direction of the shear. In a typical lateral shearing system, the wavefront is also sheared in an orthogonal direction, to obtain a measure of the wavefront slopes in the orthogonal direction. Then the slope data in the two orthogonal directions are used to reconstruct the wavefront, usually using digital processing techniques. As noted in the Hardy paper, for a 20.times.20 array of phase differences approximately 300 simultanteous equations must be solved to reconstruct the wavefront. For large-diameter light beams, wavefront reconstruction poses a significant practical problem in that a computing device of relatively high speed and capacity must be employed for effective real-time wavefront control.
Other types of interferometers used in active optical systems include ac-coupled rotating Ronchi grating interferometers, multidither interferometers, sliding reference interferometers, and ac-coupled Hartmann plate devices. All of these have specific complexities and shortcomings. The basic principles and limitations of multidither interferometers are described in the Hardy paper referred to above. As already mentioned, all of the shearing interferometers suffer bandwidth limitations because of the complexity and computation time involved in wavefront reconstruction. The Hartmann device also has significant alignment difficulties, and any of the devices using rotating elements have an instrinsincly low bandwidth.
Some active optical systems employ an interferometer of the same general type as the Twyman-Green interferometer, which was developed in about 1916 and has been described in many texts on optics. For example, "Principles of Optics" by Max Born and Emil Wolf, Sixth Edition 1980, describes the Twyman-Green interferometer with reference to FIG. 7.41 on page 304. The basic technique of the Twyman-Green instrument is to modify the well-known Michelson interferometer, which splits a light beam into two components with a semireflective mirror, reflects each component from a plane mirror, and then recombines the components to produce an interference pattern. The pattern is dependent on the path lengths followed by light rays in the two components, and one path length may be varied by moving one of the plane mirrors. In the Twyman-Green interferometer, an optical element to be tested, such as a mirror or lens, is inserted in one of the two light paths, and the resulting interference pattern is indicative of the type of aberration suffered by the component being tested.
The Twyman-Green interferometer is used, for example, in a technique proposed in a paper by N. A. Massie et al. entitled "Flow Field Testing with 64 Parallel Channel Heterodyne Interferometer," Proc. of SPIE--The International Society for Optical Engineering, Vol. 351, Wavefront Sensing, pp. 141-47, August 1982. This paper and the others cited in it disclose a device referred to as a heterodyne interferometer. Heterodyne interferometry is a technique in which phase angle differences being measured at a wavefront are carried by a high-frequency "dithering" signal superimposed on the system. The phase angle differences may then be extracted in a synchronous detection step. The principal advantage of heterodyne interferometry is that it is highly insensitive to noise signals that would otherwise distort the accuracy of the phase angle measurements.
In spite of all of these developments, there is still a need for a simple, reliable and fast technique for measuring wavefront distortion, and, in some applications, for compensating for the distortion in real time. Ideally, a wavefront sensor should be less complex than those of the prior art, and should be capable of providing a measure of wavefront distortion without complex numerical computation, to permit application to relatively large light beams. The present invention fulfills this need and provides additional advantages over prior wavefront sensing devices.